An ageing steam pipe containing asbestos exploded beneath Fifth Avenue in Manhattan early on Thursday, hurling chunks of tarmac into the air, sending out a geyser of white vapour 10 stories high and forcing the evacuation of 28 buildings. City officials said there was no major public health threat.

Five people, including three civilians, suffered minor injuries from the 6.40am blast on 21st Street, and officials warned people who may have got material on them to bag their clothes and shower immediately as a precaution.

“There was asbestos in the steam line casing,” Mayor Bill de Blasio said about eight hours after the explosion, but “the air cleared fairly quickly after the incident. ... There is no meaningful presence of asbestos in the air at this point.”

On a street near the blast site, firefighters stripped off their heavy outerwear, bagged it and entered a red decontamination tent in their gym shorts and T-shirts to take showers. Responders’ vehicles were hosed off.

De Blasio said it could take days to check and clean the buildings, which include 28 in a “hot zone” closest to the site where the blast left a crater roughly 20 feet by 15 feet (6 metres by 4.5 metres) in the street.

It was not immediately determined what caused the blast in the 20-inch (50cm) pipe. No work was being done on the pipe at the time.

Daniel Lizio-Katzen, 42, was riding his bike home when he saw the plume from the high-pressure steam explosion.

“It was a pretty violent explosion,” Lizio-Katzen told The New York Daily News. “The cars around were coated in mud. … It left a huge crater in the middle of the street.”

Brendan Walsh, 22, a senior at New York University, had just got off a train and was headed to class when he saw the plume and “a large scatter of debris”.

“I was standing behind the police line when a Con Ed worker came rushing over and screaming at police and firefighters to push everyone north because he was worried that there could be secondary manhole explosions,” Walsh said.

“Everyone – including the police and firefighters who were standing by – started moving back.”

Businesses were braced for the worst as the response dragged on and police and firefighters blocked access to buildings close to the explosion, crippling their neighbourhood and their workday. Subway trains were diverted around the blast area.

Similar explosions over the year have drawn attention to the ageing infrastructure beneath the streets of the nation’s largest city. Fire Commissioner Daniel Nigro said the pipe that blew was installed in 1932.